A 30-second silence will be held in memory of the Boston victims just before runners set off at 9.45am.

Olympic bronze medallist Kelly said: “After Boston I think the run will mean even more. I will have a little bit more determination and passion to finish.

"I know it was different but 100 days before the 2004 Athens Olympics there was a bomb there. People asked me at the time if I was going to pull out and there’s no way I would then or this week.”

Kelly, 36, making her marathon debut for Age UK and Youthnet, added: “I’m a big cry baby so I’m sure I will cry at the finish line. After everything this week it will be even more emotional.”

This Morning presenter Matt, 30, also vowed to run, saying: “This year’s race is now more than an amazing charity fundraising event. It is a message of hope, a shining example of human spirit and strength. I’ll remember that, especially on the 26th mile.”

Iwan, 39, revealed he would be wearing specially-designed socks to remember the Boston victims.

The Welsh star, running for a fourth consecutive year, said: “They will have Run For Boston written on them.

“I’ve also heard that people will put their hands on their hearts as they cross the finish line, so I will do that too. I just think you can’t let them win.”

Biking star James, 32, who is running with his brother, said his nephews would not be supporting them due to safety fears. James, who is married to singer Katie Melua, added: “I spoke to my brother, who has three little boys, to see what he wanted to do.

"As a parent you don’t want to take any risks. As an adult we can judge the risks ourselves but I understand why my brother doesn’t want to put his kids at risk.”

Welsh babe Katherine Jenkins, 32, is determined to run and is raising cash for Macmillan Cancer Support in memory of her dad Selwyn, who died of lung cancer when she was just 15.

She said: “I saw first-hand the impact cancer has on a family. The nurses who looked after him were amazing.”